The following post-em from Mike Lysell, mlysell AT comcast.net, (which I believe Mike posted under the wrong Osbern), shows who one source has for Osbern's father (I have changed to agree with Mike's source):

Jim - Carl Boyer in "Medieval English Ancestors of Certain Americans", page 104, shows Osbert (Osbern Bolbec) Giffard as the first son of Walter Giffard, born 1010. He shows a birth date of about 1040 and a date of death as "by 1095". He states "the Lord of Brimsfield, Gloucestershire, he was a Domesday tenent there in 1086 as well as in Stoke, Rockinghamshire, Elston and Orcheston [CP, 5:639c].

At this time, I've not tried to confirm this through other sources. I thought you'd be interested.

Mike Lysell

Boyer cites Vernon M. Noor’s 'Some Early English Pedigrees' as a source.

Note: CP appears to be correct in that the town is certainly now known as Brimpsfield (with a "p"), and was entered as "Brimesfelde" in the Domesday Book.

Frederick I of Luxembourg, b. c 965, d. 1019, Count of Salm and Luxembourg; m. before 995, Irmentrude of Gleiberg, daughter of Herbert I, d. 992, Count in the Kinziggau, Count of Gleiberg, by his wife Irmentrude (Herbert I was son of Udo, Count in the Wetterau, by his wife a daughter of Herbert I, Count of Vermandois. [Ancestral Roots]

General Notes: Wife - Ermentrude Gleiberg, of Gleiberg

Irmentrude of Gleiberg, daughter of Herbert I, d. 992, Count in the Kinziggau, Count of Gleiberg, by his wife Irmentrude (Herbert I was son of Udo, Count in the Wetterau, by his wife a daughter of Herbert I, Count of Vermandois). [Ancestral Roots]

Note: Turton has Ermentrude (no last name) as daughter of Mengasus, d. 1001, by Gerberge, daughter of Godfrey (no last names). Mengasus & Gerberge look to be her maternal grandparetns. I am following AR above.